1. Field of the Invention
The present invention relates to a resulfurized stainless steel with high machinability and having an improved corrosion resistance, which is especially suited to use in the field of very-high-speed machining and screw machining.
2. Description of the Background
European Patent No. 403 332 teaches a resulfurized steel with improved machinability. That document describes a process in which it is proposed, in order to improve the machinability, to introduce, into a steel having the following general composition: carbon less than 0.15%, silicon less than 2%, manganese less than 2%, molybdenum less than 3%, nickel between 7% and 12% and chromium between 15 and 25%, an amount of sulfur in a proportion of between 0.1 and 0.4%, combined with calcium and oxygen in contents of greater than 30xc3x9710xe2x88x924% and 70xc3x9710xe2x88x924%, respectively, the calcium and oxygen contents satisfying the Ca/O ratio of between 0.2 and 0.6.
In that document, the desired aim is the formation, with manganese and, in a smaller proportion, with chromium, of a manganese chromium sulfide (Mn,Cr)S which produces, in the form of specific inclusions, solid lubrication of the cutting tool during the machining operations.
It is also taught that sulfur has an unfavorable effect on the corrosion resistance. Despite this, a chosen approach is the introduction, into a resulfurized steel containing manganese sulfide inclusions, of inclusions consisting of lime aluminosilicate oxides. These oxides, most often combined with manganese sulfide inclusions, do not degrade the corrosion resistance.
Such a steel has good machinability properties in the field of conventional cutting speeds, that is to say of less than 500 m/min in turning. The steel includes associated inclusions composed of oxides of the lime aluminosilicate type with manganese sulfide inclusions. These inclusions are larger and more deformable than the sulfide inclusions by themselves.
The effect of the so-called solid lubrication of the cutting tool is thereby improved. However, the steel described in the abovementioned document has the drawback associated with resulfurized steels, i.e. a low corrosion resistance, especially pitting corrosion resistance.
Patent FR 95/04140 discloses a steel with improved machinability that can be used, on the one hand, in the field of very-high-speed machining, with cutting speeds in turning possibly exceeding 700 m/min, and, on the other hand, in the field of screw machining with 30% higher productivities than those obtained with an ordinary resulfurized austenitic stainless steel.
The resulfurized stainless steel with improved machinability that can be used especially in the field of high-speed machining and the field of screw machining has the following weight composition: carbon less than 0.1%; silicon less than 2%; manganese less than 2%; nickel from 7 to 12%; chromium from 15 to 25%; sulfur from 0.10 to 0.55%; copper from 1% to 5%; calcium greater than 35xc3x9710xe2x88x924%; oxygen greater than 70xc3x9710xe2x88x924%, the ratio of the calcium content to the oxygen content being between 0.2 and 0.6.
Although the characteristics in the field of machinability are improved by the presence of a high copper content, the corrosion resistance properties remain mediocre in this resulfurized steel.
It is taught that manganese sulfides are very hardly substituted with chromium because of a manganese content matched to the sulfur content and that their malleability, and hence their effectiveness during cutting, is thereby improved.
An object of the present invention is to provide a steel containing sulfur, in order to improve machinability, and having specific inclusions providing a substantial improvement in the field of corrosion resistance, especially pitting corrosion resistance.
The steel of the invention makes it possible to reconcile the level of machinability of resulfurized steels with having a corrosion resistance similar to that of steels of low sulfur content.
One subject of the invention is a resulfurized stainless steel with high machinability and having an improved corrosion resistance, which includes, in its composition, anorthite- and/or pseudo-wollastonite-and/or gehlenite-type lime aluminosilicate inclusions combined with CrMnS inclusions, the chromium content of which is between 30% and 70%.
In one preferred example of the invention,
the steel is a resulfurized austenitic stainless steel comprising, consisting essentially of, and consisting of the following composition by weight based on total weight:
0.01% xe2x89xa6carbon xe2x89xa60.1%;
0.01% xe2x89xa6silicon xe2x89xa62.0%;
0.01% xe2x89xa6manganese xe2x89xa60.5%;
10% xe2x89xa6chromium xe2x89xa625%;
7% xe2x89xa6nickel xe2x89xa612%;
0.15% xe2x89xa6sulfur xe2x89xa60.45%;
0.01% xe2x89xa6molybdenum xe2x89xa63.00%;
0.5% xe2x89xa6copper xe2x89xa63.5%;
0.01% xe2x89xa6nitrogen xe2x89xa60.1%;
0.0020% xe2x89xa6aluminum xe2x89xa60.0100%;
0.0005% xe2x89xa6phosphorus xe2x89xa60.050%;
30xc3x9710xe2x88x924% xe2x89xa6calcium xe2x89xa6200xc3x9710xe2x88x924%;
70xc3x9710xe2x88x924% xe2x89xa6oxygen xe2x89xa6300xc3x97xe2x88x924%;
0.20xe2x89xa6calcium/oxygen xe2x89xa60.60,
plus iron and residual elements inherent in smelting, the steel preferably containing anorthite- and/or pseudo-wollastonite- and/or gehlenite- type lime aluminosilicate inclusions combined with CrMnS inclusions, the chromium content of which is between 30% and 70%;
the composition by weight furthermore contains less than 3xc3x9710xe2x88x924% boron;
the composition by weight furthermore contains from 0.01% to 0.3% vanadium.